1. Field of the Invention
The present invention relates a RF transceiver, and more particularly, to a RF transceiver having a T/R switch circuit with high power-handling capability.
2. Description of the Prior Art
In the wireless communication system, RF front-end circuit is gradually making use of low cost and highly integrated system of complementary metal oxide semiconductor transistors (CMOS) processed products. The transceiver usually uses a common antenna, so the T/R switch circuit functions as a transmitting/receiving path for the antenna. Thus, the T/R switch circuit needs enough isolation, low power consumption, and low insertion loss. In addition, the T/R switch circuit should have high power-handling capability according to the system requirement.
Filed effect transistor (FET) is generally used for T/R switch circuit because its power consumption is almost zero. The GaAs FET is the mainstream due to its low insertion loss and high isolation, but one disadvantage is the negative control voltages. As the development of CMOS process, many RF sub-circuits are developed, such as low noise amplifiers, voltage controlled oscillators, and mixers. In view of system on chip (SOC), the T/R switch circuit produced by CMOS process is necessary. When a transistor functions as a switch, the power consumption of the transistor is zero. The state of the transistor is determined according to the channel of the transistor. When the channel is turned on, the path of the transistor from the source to the drain is equivalent to a resistor having small input impedance. On the other hand, when the channel is turned off, the path of the transistor from the source to the drain is equivalent to a capacitor having great input impedance.
Please refer to FIG. 1. FIG. 1 is a schematic diagram of a conventional RF transceiver 10. The RF transceiver 10 uses the same antenna 11 to receive and transmit signals, so the RF transceiver 10 uses the T/R switch 12 to switch. The RF transceiver 10 receives a RF signal from the antenna 11. The RF signal is amplified by a low noise amplifier (LNA) 13 and is inputted to a mixer 14 so as to generate an intermediate-frequency or baseband signal according to an oscillating frequency generated by an oscillator 15. A de-modulator 16 converts the intermediate-frequency or baseband signal to a digital signal and transmits to a baseband circuit 17. In addition, when the RF transceiver 10 sends a signal, the baseband circuit 17 transmits a digital signal to a modulator 18. The modulator 18 converts the digital signal to an intermediate-frequency or baseband signal. A mixer 19 generates a RF signal according to the intermediate-frequency or baseband signal and an oscillating frequency generated by an oscillator 20. The RF signal is amplified by a power amplifier (PA) 21 and is transmitted by the antenna 11.
Please refer to FIG. 2. FIG. 2 is a schematic diagram of a conventional T/R switch. The T/R switch can be divided into a series type and a shunt/series type. FIG. 2 shows a shunt/series type T/R switch, including four transistors M1, M2, M3, and M4. The transistors M1 and M2 are coupled in series for transmission and isolation. When the transistor M1 is turned on, the transistor M2 is turned off. When the transistor M1 is turned off, the transistor M2 is turned on. In addition, the transistors M3 and M2 are turned on at the same time. Thus, the low impedance is generated at the isolated terminal to provide a path to the ground so as to reduce the signal loss. Similarly, the transistors M4 and M1 are turned on at the same time. For the transistors M1 and M2, when the channel of the transistor is turned on, the resistance from the drain to the source becomes smaller as the size of the transistor increases. Thus, the insertion loss decreases but the parasitic effects of the substrate increases. The insertion loss decreases to a minimum and increases again because of the parasitic effects. When the channel of the transistor is turned off, the capacitance from the drain to the source becomes greater as the size of the transistor increases so as to reduce the isolation.